A number of apparatus is well known to the art for detecting gamma rays emitted by or transmitted through an object of unknown internal composition. Such emitted or transmitted gamma rays are indicative of information about the unknown composition. For example, gamma ray transmission and detection is used in medical imaging of human organs and the like by the known method of Positron Emission Tomography (PET). As another example, gamma ray transmission and detection may be used in airports and the like for examining luggage and the like, to detect concealed explosives or other nitrogenous contraband, such as drugs. Gamma ray emissions and transmissions are particularly useful in these fields, as well as others, because the higher energies of the gamma rays will more easily penetrate matter than normal light or even X-rays. Thus, such gamma ray emission or transmission can be used to image the interior of objects which are more or less opaque to normal light or to X-rays.
A very critical component of such apparatus is the gamma ray detector. Conventional gamma ray detectors determine the two-dimensional positions of gamma ray-induced light emissions in the detectors, i.e. the X and Y positions. An example thereof is the detector disclosed in U.S. Pat. No. 4,750,972, the disclosure of which is incorporated herein by reference and relied upon. These two-dimensional position detectors are quite adequate for many purposes, but, for other purposes, the lack of the third dimension position, i.e. the Z dimension of the position in the detector light guide of the gamma ray-induced light emission, creates significant problems. As an example thereof, such conventional detectors are in generally block form having perpendicular slots cut in the block so as to form a plurality of individual rectangular, elongated, light guides between those slots. Photodetectors are placed at an end of the block, and by determining, through the photodetectors, which of the light guides has been excited to produce gamma ray-induced light emission, the X and Y positions of that light emission in the plurality of light guides can be determined, as described in the above-noted U.S. Pat. No. 4,750,972. However, since that block having the light guides may be from as little as 2 cm to as much as 8 cm in length or longer, those conventional detectors provide no information as to where, along that 2 to 8 cm length of the excited light guide, the emission took place, i.e. no information of the Z position of the emission.
While the unknown Z position is not a problem in many applications of gamma ray detection, it is a significant problem in other applications of gamma ray detection. For example, the lack of information regarding the Z position of the light emission in the excited light guide is directly responsible for degrading parts of produced images in conventional apparatus, and in particular in PET cameras and some embodiments of luggage scanning devices. For example, the quality of the images made with PET cameras is generally specified by a measured image spatial resolution, which is defined as the apparent size of an image of a small test source of emitted gamma rays. Because of the lack of information regarding the light emission in the excited light guides in the Z position, the spatial resolution of existing PET cameras is much worse at the edges of an extended object, such as a human torso, than it is at the center of the object. This loss of resolution causes blurring of the image of the edges, and such blurring can obscure objects or portions of interest which might lie at those blurred edges.
Accordingly, as can be easily understood, a significant advantage could be achieved if the position of the point of gamma ray-induced light emission in light guides could be determined for all of the possible X, Y and Z positions. Having established the three-dimensional position of the gamma ray-induced light emission in the light guide, the blurring, noted above, can be largely obviated. However, a practical means of determining the Z position of the gamma ray-induced light emission in light guides has eluded the art, and the difficulty in this regard can be easily understood from even a brief consideration of conventional detectors employing those light guides.
Thus, in conventional detectors, a plurality of spatially-separated, closely adjacent, elongated, scintillating, crystal, light guides are arrayed in parallel relationship to each other and in a pattern having known X and Y positions for each of the light guides. Each of the light guides has a gamma ray receiving end, a light transmitting end and walls therebetween. A light reflective surface, usually provided by a light reflective coating, is placed on at least the walls of each of the light guides, and, usually, a plurality of photodetectors are disposed in register with one of the ends of the light guides. The photodetectors are arrayed in a pattern having known X and Y positions.
With this arrangement, when gamma rays enter the gamma ray receiving end of the light guides, under statistical probabilities, a gamma ray will, with a high degree of probability, induce a light emission in one of the light guides. Because of the reflective surface, e.g. coating, on the walls of that light guide (as well as on the walls of all of the other light guides) that light emission travels through the light guide and exits the light guide at one of the ends thereof. Since the photodetectors are in register with one of the ends and have known X and Y positions, the photodetectors, with appropriate counting means, as disclosed in U.S. Pat. No. 4,750,972, can determine, with reasonable certainty, which light guide was excited by reception of a gamma ray and, hence, produced the emitted light. Knowing which guide was so excited, establishes the X and Y positions of that gamma ray, relative to the array of light guides. Conventional detectors of this nature are well known in the art. A concise explanation of their operation is set forth in Rogers, et al, "An Improved Multicrystal 2-D BGO Detector for PET", IEEE Trans. Nucl. Sci., NS-39 1063 (1992), as well as in Dahlbom and Hoffman, "An Evaluation of a Two-dimensional Array Detector for High Resolution PET", IEEE Transaction on Medical Imaging, Volume 7, pages 264-272 (1988).
The photodetectors detect, therefore, essentially the presence of light produced in a particular light guide, upon entry thereof by a gamma ray, but the determination of the presence of light emitted in a light guide cannot provide any information as to whether that emitted light came from a position in the light guide near the light transmitting end or from a position in the light guide nearer the gamma ray receiving end, i.e. the Z position. Since, as noted above, conventional light guides may be anywhere from about 2 cm to 8 cm in length, and since conventional detectors cannot tell where along that 2 to 8 cm length the light emission took place, there is no possibility of establishing the Z position of that light emission. Without that Z position, as noted above, images, for example in a PET camera, are blurred, particularly near edges of an object being imaged.
A recent approach in the art, generally, directed to this problem, i.e. blurred images, is disclosed in U.S. Pat. No. 5,122,667 to Thompson. However, the device described in the Thompson patent applies only to small single crystal detectors, rather than an array of a plurality of crystal detectors, and is, therefore, not useful in common commercial PET cameras, which employ block detectors, as briefly described above, and which block detectors are essential for purposes of economy of manufacture. In addition, the approach of the Thompson patent is that of providing a localized surface treatment of the crystal so as to form a band which absorbs light which would otherwise be reflected, and that surface treatment is located precisely at a point which divides the crystal into two equally possible depths. While this can give some general idea of the Z position of the emitted light in the light guide crystal, this approach produces a very inaccurate Z position result having only two possible values, and the accuracy thereof is not sufficient for avoiding the imaging problem, as discussed above.
As can, therefore, be appreciated from the above, it would be a substantial advantage to the art to provide means and methods for determining the three-dimensional position of the point of gamma ray-induced light emission in a pattern of a plurality of scintillating light guides. With that three-dimensional position determined, the difficulties, noted above, with PET cameras and luggage inspection apparatus, as well as other apparatus, can be largely avoided and improved imaging can thereby be achieved.